As the amount of communication information has increased in recent years, radio communication techniques using high-frequency bands, capable of transmitting a large amount of information at high speed are attracting a lot of attention. In particular, in 60 GHz bands, any user can use up to 2.5 GHz band per radio device without possessing any license. Therefore, it is possible to perform communication at a rate equal to or higher than 1 Gbps by using such radio devices.
In general, sine the propagation loss in a free space becomes larger in high-frequency bands, antennas having a high gain are used in order to increase the communication distance. When an antenna having a high gain is used, it is very difficult to perform the alignment of the antenna because the half-width of the radiation pattern is narrow. In communication using an antenna having a high gain, it is possible to perform excellent communication by performing the communication while changing the radiation angle of radio signals, finding a radiation angle at which the received power is maximized, and using that radiation angle. When the antenna is deviated from the proper alignment, the above-described process is performed again so that the optimal communication state can be automatically maintained.
FIG. 15 shows a radio communication apparatus that automatically performs the alignment of an antenna. A radio communication apparatus 63 transmits a radio signal while changing the direction of the radiation angle. Meanwhile, a radio communication apparatus 64 measures the S/N (signal-noise) ratio of a received radio signal and transmits the measured value to the radio communication apparatus 63. The radio communication apparatus 63 calculates the radiation angle at which the S/N ratio is maximized based on the notified measured value, and then performs communication by using that radiation angle.
As a method for controlling the radiation angle of a radio communication apparatus, there is a control method using an antenna array. The antenna array is an antenna formed by arranging a plurality of antenna elements in an array. In the control using an antenna array, the radiation angle is controlled by controlling the phases of radio signals emitted from individual antenna elements. Note that a phase shifter is used for the phase control of a radio signal.
FIG. 16 is a block diagram showing a typical configuration of a phase shifter. This phase shifter includes a 180-degree divider 65, 0-to-90-degree phase shifters 66 (66-1 and 66-2), and a combiner 67. The 0-to-90-degree phase shifters include 90-degree dividers 69 (69-1 and 69-2), gain-variable amplifiers 70 (70-1 to 70-4), combiners 71, and resistors 68 (68-1 and 68-2).
Each of the 90-degree dividers 69 is composed of a 4-terminal circuit. The isolation ports of the 90-degree dividers 69 are connected to the ground through the resistors 68. Each of the resistors 68 preferably has a resistance value equal to the characteristic impedance of the transmission line. Typically, a 50Ω-resistor is connected.
A signal input to the phase shifter shown in FIG. 16 is divided into four radio signals having phases of 0, 90. 180 and 270 degrees by the 180-degree divider 65 and the two 90-degree dividers 69, and these four signals are input to the gain-variable amplifiers 70 (70-1 to 70-4). The gain-variable amplifiers 70-1 to 70-4 change the amplitudes of the input signals. Each of the combiners 71 (71-1 and 71-2) combines two signals input from the respective gain-variable amplifiers 70 and outputs the combined signal to the combiner 67. Similarly, the combiner 67 combines signals input from the combiners 71-1 and 71-2.
Note that the amplitude change rates by the gain-variable amplifiers 70-1 to 70-4 are changed according to the desired phase angle. For example, when a phase of or close to 90 degrees with respect to an input signal is to be set, one of the gain-variable amplifiers 70 to which a signal having a phase of 90 degrees is input amplifies this signal and the other gain-variable amplifiers 70 decrease the amplitudes of their input signals. With the operation like this, the phase control amount of the phase shifter shown in FIG. 16 can be set to an arbitrary angle within the range from 0 degrees to 360 degrees.
However, depending on the use conditions of the radio device, the number of antenna arrays, and/or the like, the phase control amount of the phase shifter does not necessarily have to be set to an arbitrary angle. For example, Patent literature 1 proposes a phase shifter that is used for the cases where the phase is controlled in steps of 90 degrees. However, when this phase shifter is used, the same number of quadrature modulators as the number of antennas are necessary. Therefore, although depending on the phase shifter, it is very difficult to perform an IQ mismatch correction for individual quadrature modulators. As a result, the process becomes easier when the phase is controlled in a radio signal band than when the phase is controlled in a baseband signal band by using the phase shifter disclosed in Patent literature 1.